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UBTU-22-255060 Ubuntu 22.04 LTS SSH server must be configured to use only FIPS-validated key exchange algorithms.

DISA STIG library V2R10
All changes in this version
first version loaded CAT II

Maps to AC-17(2) via CCI-000068

Title

Ubuntu 22.04 LTS SSH server must be configured to use only FIPS-validated key exchange algorithms.

Control

Verify that the SSH server is configured to use only FIPS-validated key exchange algorithms by using the following command: $ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH 'kexalgorithms' /etc/ssh/sshd_config:KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 If "KexAlgorithms" does not contain only the algorithms "ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256" in exact order, is commented out, is missing, or conflicting results are returned, this is a finding.

Discussion

Without cryptographic integrity protections provided by FIPS-validated cryptographic algorithms, information can be viewed and altered by unauthorized users without detection. The system will attempt to use the first algorithm presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest algorithm available to secure the SSH connection.

Related controls

CCI-000068

Fix

Configure the SSH server to use only FIPS-validated key exchange algorithms. Add or modify the following line in the "/etc/ssh/sshd_config" file: KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 Restart SSH for the changes to take effect: $ sudo systemctl restart ssh